There had been pressure for Mugabe - who remains president in the coalition - to step down altogether, and questions remain about whether a partnership can work after a long history of state-sponsored violence against Tsvangirai and his supporters.

While Mugabe recently declared "Zimbabwe is mine," he went further Wednesday than many would have expected. He stood to face Tsvangirai as an equal in a white tent on the grounds of the presidential palace.

Regional leaders watched from the tent and Zimbabweans across the country watched on state TV as Tsvangirai raised his right hand and declared: "I will well and truly serve Zimbabwe in the office of prime minister of the Republic of Zimbabwe, so help me God."

Both Tsvangirai and Mugabe were relaxed and smiling during the brief ceremony, which also included the swearing-in of Tsvangirai's deputies, Arthur Mutambara of a breakaway opposition party and Thokozani Khupe of Tsvangirai's party.

Together, Tsvangirai and Mugabe will be under pressure to act quickly to alleviate the suffering of impoverished Zimbabweans. The country's economic collapse - for which Tsvangirai holds Mugabe responsible - has led to the world's highest inflation rate, left millions dependent on international food aid, and caused a cholera outbreak that has killed some 3,400 people since August.

Neighboring leaders who pushed for the coalition said that once the two men had joined in the unity government, they would overcome mutual mistrust and work together for the good of their country.

Mugabe and Tsvangirai have clashed repeatedly since the decade-old opposition Movement for Democratic Change emerged as the most serious threat to the ruling regime since independence.

Tsvangirai has been beaten and jailed by Mugabe's security forces. In 2007, police attacked him after he held an opposition meeting the government had banned. Images shown on news broadcasts around the world of his bruised and bloodied face came to symbolize the challenges his movement faced.

Mugabe, who turns 85 on Feb. 21 and has been in power since independence from Britain in 1980, has in the recent past treated the 56-year-old Tsvangirai as a junior partner at best, often not bothering to hide his contempt.

But Tsvangirai won the most votes in the first round of presidential election held almost a year ago, and withdrew from a June runoff only because of attacks on his supporters.

Tsvangirai's party, the Movement for Democratic Change, also broke ZANU-PF's lock on parliament in March 2008 elections for the first time since independence.

The European Union's top aid official, Louis Michel, called the swearing in of Tsvangirai as prime minister welcome news, but said the new power-sharing government "has a heavy responsibility" to ensure action is taken quickly.

The coalition agreement calls for the government to make its priority reviving an economy the opposition accuses Mugabe of destroying through corruption and mismanagement. The world's highest inflation rate has left millions of Zimbabweans dependent on international food aid to survive.

Even if the factions can put aside their differences, they cannot do much without foreign help. The world's main donor, the United States, has made clear the money won't flow if Mugabe tries to sideline Tsvangirai.

The president of neighboring South Africa, Kgalema Motlanthe, told Parliament on Tuesday that the swearing in "is a vindication that our approach to the crisis of Zimbabwe all along has been correctly, despite skepticism in certain quarters." Motlanthe called on the international community to turn its attention now to Zimbabwe's humanitarian crisis.

Former South African President Thabo Mbeki, who mediated the unity deal, had stuck to a strategy of quiet diplomacy on Zimbabwe for years despite criticism that the approach amounted to appeasing Mugabe.

On Tuesday, Tsvangirai announced that one of his most senior aides, Tendai Biti, would head the Finance Ministry. Days earlier a judge had shut down a treason trial for Biti, who had faced a possible death sentence.

Tsvangirai's party also holds the Health Ministry, another key post given the country's cholera epidemic. The rapid and unusually deadly spread of the disease has been blamed on the collapse of Zimbabwe's health and sanitation infrastructures because of lack of funds for maintenance.

The unity government's agenda includes preparing for new elections, expected in a year or two. Media restrictions will have to be lifted and other steps taken to ensure the elections are free and fair, after several ballots marred by violence, intimidation and manipulation blamed on Mugabe's party.

Tsvangirai on Tuesday called for political detainees to be released before he is sworn in as prime minister, but did not say what he would do if they were not. Human rights groups say tortured detainees are on the verge of dying in jail.

Some Tsvangirai allies say he never should have agreed to serve as prime minister in a government that left Mugabe president. Mugabe, meanwhile, has been under pressure from aides in the military and government who don't want to give up power and prestige to the opposition.

Problems emerged almost as soon as the factions agreed to their partnership in September. Mugabe unilaterally claimed all the most powerful Cabinet posts for ZANU-PF, including the ministry in charge of the police accused of attacking dissidents.

Regional leaders then decreed the police ministry would be alternated between ZANU-PF and MDC politicians, only one of several compromises that raise questions about how the unity government can practically work.

At first, Tsvangirai said he would not join the government until a more equitable Cabinet allotment was worked out, and until attacks on his supporters stopped. Regional leaders met five times to pressure Mugabe and Tsvangirai to move forward. Tsvangirai gave in on Jan. 30, agreeing to join the government now and resolve outstanding issues later.

Online Public Information File

Viewers with disabilities can get assistance accessing this station's FCC Public Inspection File by contacting the station with the information listed below. Questions or concerns relating to the accessibility of the FCC's online public file system should be directed to the FCC at 888-225-5322, 888-835-5322 (TTY), or fccinfo@fcc.gov.